The present invention relates to a seat belt apparatus for a person's safety for a vehicle such as an automobile, and more particularly, to a seat belt apparatus which prevents a belt in an automobile from sudden loosening and varying the effective length of the belt.
In a seat belt apparatus for an automobile, it is common, during either a sudden stop, sudden acceleration or during a crash, to lock a seat belt so as to restrain a person on his or her seat, and in a situation that is not dangerous, to loosen the belt to accommodate a person's movement so as to make him or her feel comfortable during movement. Such a seat belt apparatus usually comprises a spool for rewinding the belt, a ratchet wheel and a pawl rotating along with the spool, and a spring for pulling the pawl directly or indirectly, so that, in the aforementioned dangerous situations, the pawl operates to prevent to loosening of the belt. For a method for operating the pawl, there is a well-known web sensing method for stretching the spring over a predetermined acceleration speed by sensing the speed at which the belt is loosened.
In the web sensing method, a timing for the pawl to lock the ratchet wheel is very important. If the timing is late, the belt cannot execute an appropriate function, whereas, if the belt is too sensitive, it may inconvenience the person. Therefore, it is required to match the resilience of the spring for sensing the drawing-out acceleration speed at which the web of the belt is drawn out to the predetermined drawing-out acceleration speed of the belt. However, in the conventional apparatus, since the installation position of the spring is designated at one place, its resilience can not be precisely controlled. Accordingly, this involves a problem in which the locking time of the belt becomes imprecise due to assembly tolerance.
When the belt is excessively loose or a child's seat is fixed, and then a dangerous situation happens unexpectedly, the belt cannot also function appropriately. Therefore, it is necessary to prevent the belt from loosening excessively.